1. Field of the Invention
This invention relates generally to the field of robotics and, more specifically, to overload detection devices typically mounted between the end of a robot arm and user application specific tooling. Such devices are generally designed to detect excessive loading, and in response thereto to interrupt robot arm movement and minimize damage to the robot mechanism and the tooling.
2. Background Information
Today, robotic equipment is used in many industries for a wide variety of applications. User tooling is normally attached to the mechanical wrist or end of a robot arm, via a robot tool mounting faceplate. In order to sense tooling encountered obstructions or overlimit conditions due, for example, to programming, servo errors or misplaced workpieces, overload detection devices have been developed. The primary purpose of such devices is to quickly detect and relieve excessive moment or torque loads applied to the robot tool mounting faceplate, and to simultaneously produce a signal which can be used to stop robot arm movement to prevent or minimize damage to the robot mechanism and/or robot work cell tooling and fixtures. Ideally, upon overload detection, the normally rigid coupling provided between the robot tool mounting faceplate and tooling by the overload detection device, becomes compliant to avoid damage prior to cessation of robot arm movement. After being "tripped", the device is desirably able to be "reset" to its original position with a high degree of repeatability in order to avoid the need to revise the robot program.
Since many types and sizes of robots exist, and normally the potential users' tooling and gripper configurations vary widely, a method is needed to vary the pre-load on the overload detection device so that it will quickly operate (i.e. produce a "stop" signal and become compliant) only after a certain torque or moment threshold load is exceeded.
Commercially available devices currently exist which are intended to provide such overload detection and protection. However, these devices exhibit certain disadvantages which limit their usefulness, flexibility, and ease of application.
Many of the existing devices utilize coil springs to generate a threshold force to resist the normal inertial and tool mass loads applied to the robot face plate during robot operation. For larger robots, the required number of springs and support features add significantly to the overall mass and physical size of the device. For most applications, the proper quantity, type and position of the springs is difficult to calculate, due to the large number of variables which exist. In addition, changing the device characteristics in the field is difficult and time consuming. For these reasons, the springs are usually installed by the manufacturer of the overload detection device. In order for the manufacturer to choose the proper spring complement, he must rely on the purchaser to supply data regarding the torque and moment values which must be resisted without tripping the device. The correct values are very difficult to determine, especially before the final design of the tooling is completed and tested In addition, the "correct values" change depending upon the acceleration of the robot and the position of the center of mass of the tooling relative to the robot axes. The acceleration of the robot and the position of the center of tool mass are constantly varying in most robot applications.
Furthermore, the trip-point of a spring-loaded type device is difficult to adjust if the design or configuration of the tooling is changed, as through the use of a robotic tool changing system (reference, for example, U.S. Pat. No. B1 4,664,588) or unforeseen required tooling modification. Electrical cables and hoses connected to the tooling also add additional unpredictable and variable loads to the robot arm.
Others have attempted to overcome these disadvantages by utilizing either springs in conjunction with air cylinders, or air cylinder type devices alone which allow a change in trip-point through a change in input air pressure to the device. One such prior art device described in U.S. Pat. No. 4,714,865 utilizes an air pressurized flexible bladder to generate the resistive force required.
In the air cylinder type devices some form of sensor is used to sense a relative displacement between the portion of the device attached to the robot faceplate and the portion to which the tooling is attached. This signal is then, normally, first processed through an electronic signal conditioner which then signals the robot to stop, and sometime to operate a valve which allows the pressure in the cylinder(s) or bladder to be relieved through a hose connecting the pneumatic valve to the overload detection device This series of events requires a relatively long time to occur, so that typically the compliance of the device is exhausted before robot arm motion can be completely halted, resulting in potentially damaging forces being transmitted through the robot structure and tooling components. In addition, the material of construction of the bladder-type device is subjected to substantial physical and sometimes chemical abuse which results in a relatively short service life in a typical factory environment.
Many of the earlier overload detection devices also lack good repeatability, either initially, or in some cases, after a relatively low number of "trip" cycles. Good repeatability is important since devices having poor repeatability require the robot program taught-points to be taught again, a generally unacceptable time-consuming process.
A robotic overload detection unit utilizing a pneumatically pressurized chamber to provide overload relieving capability for a robot and tooling was previously developed by the assignee of this application. In this earlier design, no air bladder, springs or air cylinders were required to provide the "clutching" (i.e. moment and torque resistive) force. Instead, an elastomeric o-ring sealed chamber was used. When an overload occurred, the o-ring seal was instantly broken, resulting in an immediate loss of resistive pressure in the chamber. The chamber pressure loss was sensed by a non-removable built-in pneumatic pressure sensor which was normally wired into the robot "Emergency Stop" circuit or other "Stop" circuit. By varying the pneumatic pressure in the chamber, the overload threshold of this device could be easily adjusted to suit the particular operating environment conditions. By utilizing a variable pressure programmable air regulator, the robot controller could dynamically change the chamber pneumatic pressure while the robot program was running.
The sealed pneumatic chamber device was sensitive to various overload conditions including moments (tipping about a z-axis), torques (rotation about the z-axis), lateral (sideways with respect to the z-axis) forces, and compressive (along the z-axis in the direction of the tool mounting faceplate) forces. The only motion that would not break the seal would be an overload condition that was a purely tensile axial force (i.e. along the z-axis in the direction away from the faceplate) with absolutely no other loading on the unit. In virtually any robotic application, this pure axial "pull" is extremely unlikely to occur.
The sealed pneumatic chamber unit is also very fast acting since the air pressure is instantly relieved by the overload force without the need for air to flow through restrictive hoses or conventional valve components. Thus, this earlier design overcame many of the shortcomings of the prior art devices Nevertheless, it too was susceptible to further improvement.
In the earlier design, the chamber sealing element was a conventional o-ring inserted into a machined groove. The o-ring groove tended to weaken the member into which it was machined due to the reduction in cross-section under the groove. If the thickness of this member was increased to compensate for this, the weight and overall height of the unit was undesirably increased. In addition, the o-ring itself proved difficult to retain in the groove due to the forces generated by the rush of air across the o-ring during the "tripping" process. A truncated "V" groove which was difficult to machine, and an adhesive, which was difficult to apply consistently were needed to retain the o-ring in the groove. The o-ring seal was also subject to deterioration due to contaminates present in the pressurizing air and materials and vapors used in the wide range of applications in which the unit could be utilized. Other difficulties encountered with the use of an elastomeric seal include deterioration of the elastomer and problems associated with maintaining good repeatability and ease of assembly caused by the relatively wide dimensional variations associated with elastomer seal manufacturing techniques.
The earlier design also employed a relatively complicated multiple staked ball bearing-detent arrangement for initial registration and compressive load bearing. Proper "Z" axis adjustment of the multiple locating elements was difficult to achieve due to parts manufacturing tolerances and normal o-ring manufacturing tolerance variations. The relative position of the elastomeric seal to the locating elements also created a locating element sealing problem because the locating elements were subjected to the chamber pneumatic pressure. A liquid sealer was required to seal the locating elements against air leaks and maintain their position.
In the prior design, an electrical switch and actuator were placed inside of the pressurized pneumatic chamber so that only the wires to the switch had to be sealed where they exited the pressure chamber. The switch was actuated by the side force of a chamfered piston against the switch actuator as the piston was forced downward by chamber pressure, and was released by upward movement of the piston caused by an imbalance in force between a piston return spring and the decreasing force generated by the falling pneumatic chamber pressure when the unit was "tripped". Unfortunately, in this earlier design, if the switch or electrical cable were damaged in operation, neither was field repairable or replaceable. Similarly, the switch operating point was not adjustable, especially after the unit was assembled. Also the switch actuation and reset points were not necessarily consistent from unit to unit due to the variable frictional forces generated by the force of the switch actuator button against the side of the piston. Moreover, a very low pressure could not be used to operate the switch since the piston return spring had to have a spring rate great enough to reliably return the piston even if the unit did not see a pressure reduction cycle for an extended period. The close fit between the piston and the piston bore also made it susceptible to sticking due to contaminates present in the chamber pressurizing air.
A need thus exists for an improved overload detection device which not only overcomes the problems of lack of durability, lack of flexibility, poor repeatability and slow reaction time exhibited by conventional overload detectors but also enhances the manufacturability, operational characteristics and field serviceability of the described earlier design.